The present invention relates to a semiconductor structure, and more specifically, to a material stack which is capable of providing a shift in flatband and threshold voltages in high-k gate stacks and CMOS devices, particularly, p-FET devices.
The use of silicon-germanium (SiGe) as the channel of a p-FET device has been shown to reduce the flatband and threshold voltage to the p-FET band edge. The shift in flatband and threshold voltage has been associated with the band-gap modulation of the SiGe layer with respect to silicon. However, the total shift that is obtained in these devices is not consistent with only band-gap narrowing.
Several methods have been employed to control the threshold voltage and flatband voltage in p-FET devices having a SiGe layer. One method includes increasing the Ge concentration of the SiGe layer. Another method includes increasing the thickness of the SiGe layer. Several problems may occur using these methods. One problem is that the increase of the Ge concentration limits the thickness of the growth layer to obtain a defect free film. Furthermore, the increase in thickness of the SiGe channel limits the Ge concentration that can be obtained. Therefore, the maximum voltage shift is limited by maximum tolerance to defects during manufacturing of the p-FET device.
In view of the above-mentioned problems, there is a need for providing a method and structure capable of controlling flatband and threshold voltages in a high-k metal gate stack by eliminating the use of a SiGe layer, and introducing a Ge material layer and tuning the interface, thickness and location of the Ge material layer in the high-k metal gate stack.